1. Field of the Invention
The present invention relates to a developing device for visualizing an electrostatic latent image formed on a photosensitive drum by supplying developer, and to a developing roller used in the developing device.
2. Description of the Background Art
In an image forming apparatus such as a copier, a laser printer or a facsimile apparatus, an electrostatic latent image is formed on a photosensitive drum and is visualized, i.e. developed by supply developer to the photosensitive drum. A developing device is incorporated into the image forming apparatus in order to perform this developing operation. As an example of the developing device of this type, the one for visualizing an electrostatic latent image by developer comprised of, for example, single-component toner is known.
This developing device is provided with a developer container for storing the developer, a developing roller for supplying the developer in the developer container to the photosensitive drum and a developer restricting means which is brought into contact with the outer circumferential surface of the developing roller prior to the supply of the developer to the photosensitive drum to restrict a thickness of a developer layer held on the outer circumferential surface within a specified range. Among these elements, the developing roller has silicone rubber as a roller base material. A primer layer is formed on the outer circumferential surface of a roller main body made of this roller base material by spraying, and a top coating layer is formed on the outer circumferential surface of the primer layer. Thus, the developing roller has a so-called three-layer structure.
The developer restricting means has been conventionally made of a metal plate of, e.g. stainless steel or a glass plate, and the leading end thereof is brought into contact with the outer circumferential surface of the roller main body to make the thickness of the developer layer to be fed to the photosensitive drum thinner and uniform along the entire width of the developing roller.
However, the above developing roller is relatively expensive since it uses silicone rubber as the roller base material and the primer layer, which functions as an intermediate adhesive layer, needs to be always provided because the top coating layer cannot be directly formed on the roller main body. Accordingly, there has been a demand for less expensive developing rollers. In order to reduce the production costs of the developing roller, it may be considered to make a roller main body of acrylonitrile-butadiene rubber (hereinafter, "NBR") and use this roller main body as a developing roller without forming a top coating layer thereon as in a process cartridge disclosed, for example, in Japanese Patent Publication No. 2675286. However, developer (toner) adheres to the outer circumferential surface of the developing roller, with the result that a satisfactory developing operation cannot be performed.
Further, in order to perform a satisfactory developing operation, much attention needs to be given to the developer restricting means to be brought into contact with the outer circumferential surface of the developing roller. This is because the developer restricting means changes the charged state of the developer since it is held in contact with the outer circumferential surface of the developing roller while being biased in order to restrict the thickness of the developer layer as described above and is brought into contact with the developer when the thickness of the developer layer is restricted to thereby charge the developer in a specified state. However, it has been absolutely unknown how the developer restricting means should be used in relation to the developer in order to realize a satisfactory developing operation. Particularly, if the developer restricting means is not properly selected in relation to the developer, various inconveniences including fogging occur, thereby reducing the quality of an image.
In order to perform a satisfactory developing operation, not only the relation of the developer restricting means to the developer, but also the relation thereof to the developing roller needs to be studied. No sufficient studies have been made on this point thus far.